CA1108940A - Marine craft - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

One or more vanes of a rigid or flexible material are attached to the underside of a liferaft or other towable marine craft. The vane or vanes are suspended from the craft at an angle of 90° or less relative to the direction of towing.

Description

This invention relates to marine craft, and particularly but not exclusively to marine craft made wholly or partly from water-impermeable, flexible sheet material.
Such craft includes liferafts, dinghies and flexible barges.
The present invention will be described herein with particular reference ~o inflatable liferafts, but is not intended to be limited thereko.
Inflatable liferafts are now well known and are carried on many vessels. Such raft~ generally comprise one or more inflatable buoyancy tubes arranged in a circular or ellipscidal plan form and having a flexible floor and a flexible roof attached to opposite sides of said tube or tubes. The roof may be supported by means of inflatable struts or arches.
When required for use, the rafts are inflated on or near to the vessel being evacuated and are then boarded.
It has been the practice to encourage boarders to maintain the location of the raft close to the position at which it was b~arded so a~ to facilitate subsequent search operations.
The success~ul outcome of a ~earch would r~sult in the raft passengers being transferred to a conventional vessel.
More recently it has bsen proposed that liferafts should be taken in tow by a powered vessel and the passengers landed ashore. It has, however, been found that many commercially available inflatable liferafts are not amenable to towing at or above certain critical ~peeds. 'Fhe critical speeds will depend upon the type of liferaft to be used and tend to be influenced by prevailing ~ea conditions. ~hey

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are ~uite low, for example less than 3 to 4 knots.
At the critical speed, a liferaft exhibi~s a tendency to nose-dive and an irregular, undulating motion occurs. In an observed situation, the bow of a towed raft repeatedly dived beneath the water and the raft became water-filled.
It remained afloat due to its reserve buoyancy but its efficiency as a life-saving apparatus was substa~tially reduced.
The phenomenon described in the immediately-preceding paragraph is similar to those described as l'hogging'l and "porpoi~ing" with reference to inflatable boats. It is believed to result from viscous drag on the underside of the flexible floor. At the critical speed, drag causes tensile forces to develop which exert a tensile load on the floor in the opposite direction to~the towing force. One effect of the tensile load is the creation of a bending effect in the region where the towing force i9 applie~. Arching occur9 in the buoyancy tubes and the bow deflects downwards.
A possible remedy would be to design a liferaft having a more "boat-like" hull. Such a remedy would, however, be prohibitively expensive and would create problems in other aspects of liferaft design. It would al80 be in-applicable to the many thousands of rafts already in service throughout the world~
It is an object of the pre~ent invention to provide a means of improving the towing characteristics of a lieeraft, said means being capable of incorporation both in lieerafts under con~truction and in already-made liferafts.

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, The invention seeks to improve the towing performance of marine craft by introducing a resistance to water movement under the craft.
Thus, in one embodiment, the present invention provides a towable, inflatable marine craft having a generally ~lat underside, means to deflect upwardly a portion of the water passing thereunder and means to tow said crat, said deflection means comprising at least onevane attached to the underside of said craft, each said vane having a surface of greater dimension and a surface of lesser dimension and being attached to the underside of said craft so that a pocket is formed between the underside of said craft and the surface of greater dimension of said vane, said towing means being attached to each said vane, whereby the surface of greater dimension of said vane makes an angle of 90 or less with the underside of said craft, relative to the direction of towing.
The invention will,be further described, merely by way of example, as applied to an inflatable lieraft and with reference to the accompanying drawings, in which:
Figure 1 is a schematic cross-sectional view of a known inflatable liferaft;
Figure 2 shows ~he liferaft of E~igure 1 under towing conditions;
Figure 3 shows an inflatable liferaft having a vane attached to its underside;
Fig~res 4 and 5 show two ways o incorporating towing means in the liferaft of E'igure 3.

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Referring now to E'iyure 1, the liferat comprises two, generally annular, superposed buoyancy tubes (1), a flexible floor (2) and a canopy (3). As shown in Figure 2, a to~1ing force is exerted in the direc~ion of arrow A by means o a painter or similar line~ Viscous drag is set up at the interface B resulting in a tensile force, indicated by the arrow C, in the floor material~ At critical speeds, the bow of the raft is forced downwards as indicated by the broken lines.
Referring now to Figure 3, a vane 4 is attahhed by its upper edge to the underside of the raft floor. The lower edge of the vane is suspended (by means not sho~n) at a point some distance below the raft so as to define an angle between the vane and the raft. When the raft is towed in the direction of arrow A, the vane will tend to act as a brake.
It is therefore surprising that, at critical speeds, the vane 4 substantially improve the towing charac~eristics of a raft.
In particular it effectively prevents the "nose-diving"
phenomenon by causing the bow to deflect upwards instead of downwards. This beneficial eff~ct is believed to arise from a change in direction of the water force effected by the vane 4 and shown by arrow D. It appears that lateral movement of the water is converted into generall~ vertical movement. At speed, the effect is comparable to that produced by a continuous, lubricatad, inclined plane. A fuxther benefit is that the turbulence cxeated aft of element 4 produce~ a lower viscous drag than would occur with a relativel~ uniform, l~teral movement of water~

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In Figure 4~ a vane 4 is provided at its lower sdge with one or more towing lines 5. Each line is attached at its distant end to a towing vessel (not shown). A further line 6 is attached at one end to the raft and at its other end to towing line 5.
In Figure 5, a towing line 5 is attached to an intermediate line 7, one end of which is attached to the raft and the other ~o the edge of vane 4.
It will be apparent that the invention can take various forms and is applicable to various ~ype~ of marine craft. Vane 4 may, for example, comprise a continuous, imperforate web, attached to the underside of a marine craft.
Alternatively, the web may be incorporated in a towing array adapted for attachment to a marine craft.
The vane 4 may also be perforated or otherwise adapted so as to deflect only part o the water flow. It may also be so shaped as to direct the water flow as desired and may be provided wi~h means to vary its characteristics.
When applied to craft made wholly or partly from flexible materials, the vane may be constructed rom one or more flexible materials. In the case of a liferaft, such a construction would not significantly aff~ct the requirement that the raft be suitable for compact packing in a container.
In the case of craft made from rigid materials, the vane may be made from one or more rigid materials.
Alt~ough, in the foreyoing e~ample, reference ha~
been made to a single vane 4, it is wikhin the ~cope of the invention ko provide a pluraliky o such vaneR.

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Similarly, although reference has been made to a single towing line 5, it is within the SCOp8 o the invention to provide a plurality of such lines, `, ' ~ , . :
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Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A towable, inflatable marine craft having a generally flat underside, means to deflect upwardly a portion of the water passing thereunder and means to tow said craft, said deflection means comprising at least one vane attached to the underside of said craft, each said vane having a surface of greater dimension and a surface of lesser dimension and being attached to the underside of said craft so that a pocket is formed between the underside of said craft and the surface of greater dimension of said vane, said towing means being attached to each said vane, whereby the surface of greater dimension of said vane makes an angle of 90° or less with the underside of said craft, relative to the direction of towing.

2. A craft according to Claim 1 in which each vane comprises a continuous imperforate web.

3. A craft according to Claim 1 or 2, in which each vane is perforate.

4. A craft according to Claim 1, in which each vane is made from one or more flexible materials.

5. A craft according to Claim 1, in which each vane is made from one or more rigid materials.